Electric heating element and method of making the same



Aug. 24, 1937. R. REICHMANN ELEQTRIC HEATING ELEMENT AND METHOD OFMAKING THE SAME Filed July 14, 1933 N A k H \\\w NW SE 4/ J 7 v RAW Jday Patented Aug. 24, 1937 UNITED STATES PATENT OFFICE ELECTRIC HEATINGELEMENT AND METH- OD OF MAKING THE SAME tion of Germany Application July14, 1933, Serial No. 680,455 In Germany July 15, 1932 2 Claims.

My invention relates to electric heating elements and to a method ofmaking the same. It has hitherto been possible to construct electricresistance furnaces for high temperatures which 3 may be operated in areliable manner only at temperatures up to 1100 C. without the use ofany particular neutral gas. Even the highly refractory metals in contactwith air oxidize very rapidly at a high temperature so that the resistormembers which are, as a rule, thin are subjected to a rapiddeterioration. It is true, that by the use of platinum as resistancematerial high-er temperatures may be obtained than is the case a withother materials. Platinum, however, has

not come into extensive use owing to its prohibitive price and alsobecause it has the tendency to volatilize at a high load and is,therefore, subject to recrystallization so that platinum as heatingresistor has but a limited g durability.

Furthermore, it has already been proposed to manufacture heatingelements for high temperatures by surrounding the heating resistor withfinely granulated argillaceous earth, placing it 25 into a mold andslntering the surrounding argillaceous earth to a firmly compactprotecting sheath by passing an electric current through the conductor.However, it is not possible to provide and to ensure in this manner aperfect 30 sintering of the argillaceous earth particularly in the caseof a sufficient thickness of the protecting sheath required in any givenstructure. Moreover, the powder loosely distributed over the I heatingelementdoes not sinter so firmly as to 3.; result in a gas-tight body.Consequently, the heating elements cannot be used for heating liquids orfuse baths, from which detrimental gases develop. It is not evenpossible to manufacture the protecting sheath in such a manner 40 as totightly fit the heating resistor. Either an interstice results, wherebythe heat transfer suffers and the entrance of air or gas at the endsofthe sheath is unavoidable or the sheath can'not contract to thedesired extent during the sintcr- 45 ing and the result is the formationof cracks.

According to my invention a perfect protection for heating resistors isobtained if the resistance material which may, as a rule, consist of ametal or a metal compound is surrounded according to 50 ceramic methodswith at protecting sheath of refractory material which does not undergoany reactions with the heating resistor and which sinters together withthe resistor in the baking furnace in an inert atmosphere at atemperature 55 above 1600 C. until the protecting sheath is completelycompact and is firmly applied to the heating resistor.

By ceramic methods such methods are to be understood which are employedin the ceramic industry for the manufacture of shaped bodies 5 whichconsist, for instance, in moistening the mass to be treated or inrendering it moldable or plastic by adding binding agents thereto andmolding it by pressing or forming or converting it into a moldable slipby adding an electrolyte.

Highly refractory metallic oxides, particularly the oxides of aluminum,beryllium, chromium, manganese, magnesium, zirconium, hafnium ormixtures or compounds thereof are particularly suitable for use asmaterials for the protecting sheath. The materials must be substantiallyfree of argillaceous substances and such containing silicic'acid inorder that no reactions occur with the resistance material. Theprotectin sheath is applied, for instance, by introducing the freelysuspended resistor member into an absorbing mold of plaster and bypouring into the mold a slip activated with diluted ac d. Thin resistormembers having a diameter less than 2 mm. may be directly embedded intothe mass of the protecting sheath. A completely gas-tight sheath firmlyapplied to the resistor member is obtained after' the sintering. In thecase of resistors of larger diameter, the resistor is first surroundedby a temporary intermediate layer or coating prior to the sintering andthen the protecting sheath is applied thereto. Wax, paraffin, lac,cellulose, silk, cotton or the like are suitable substances for such anintermediate layer. The thickness of this intermediate layer ortemporary coating is so chosen that the space occupied thereby is justenough to permit normal shrinkage of the refractory casing whensubjected to the sintering temperature. The ends of the resistorsprojecting from the protecting sheath are shielded during the sinteringprocess.

As heating conductor any highly refractory metals, such as tungsten,molybdenum or alloys of tungsten or molybdenum may be employed. An alloyof tungsten and molybdenum has proved to be particularly suitable sinceits coefiicient of expansion is approximately equal to that of theprotecting sheath consisting in this case preferably of beryllium oxide.Consequently, detrimental changes in the heating resistor which mayresult in operation owing to a non-uniform expansion or contraction ofthe heating resistor and protecting sheath are avoided.

In the accompanying drawing some emb0d1 ments of my invention arediagrammatically shown.

Fig. 1 illustrates a longitudinal view of a heating element partly insection;

Fig 2 shows a longitudinal view of a modified form of a heating elementpartly in section, and

Fig. 3 shows a perspective diagrammatic view of a muffle with a heatingelement embedded in the wall thereof.

In Fig. 1 the resistor member I consists, for instance, of an alloy oftungsten and molybdenum. The ends of the resistor wire I are somewhatenlarged as indicated at 2 and joined with other enlargements 3 whichpartly project from the protecting sheath so that the supply conductorsmay be connected thereto. The enlargements are, moreover, provided witha spirally wound wire 4 of the same material. The whole is surrounded bythe highly sintered protecting sheath 5. The material of the protectingsheath penetrates into the interstices of the spirally wound wires andthereby ensures a perfect sealing at the ends of the sheath.

As shown in Fig. 2'the outer ends 3 of the resistor are tapered withinthe protecting sheath 5 to the operative heating cross-section of theresistor portion I. Consequently, the heat is substantially developedonly in the thin resistor portion I, thence it is transferred to theprotecting sheath 5, whereas the enlarged ends 3 remain cooler'and theportion of the protecting sheath surrounding said enlargements are lessheated. Furthermore, the resistor and the protecting sheath are in firmengagement with each other to form a gas-tight heating unit. Contactresistance between the resistor and the ends extending outwardly isavoided.

It is understood that other forms of heating elements, e. g., forimmersion in liquids or fuse baths and the like, may be alsomanufactured.

Furthermore, it is also possible to construct plate-shaped heatingelements in accordance with this disclosure which may be used, forinstance, for cooking plates. In this case the completely tight sinteredprotecting sheath may be brought into direct contact with the objects tobe heated. As a result of thetight sintering of the protecting sheath apenetration of moisture into the heating conductor is renderedimpossible. In

the mufiie shown in Fig. 3 the spirally Wound resistor l is completelyand gas-tight embedded inthe wall 5 of the muflle in the form of azig-zag line, The material of the mufile contacts directly with theheating element. The supply lead connections 3 are arranged at the rearside of the muflle. The mufile is surrounded in the usual manner with aheat insulation and closed at the front side by a door. The gas-tightembedded heating resistors are completely protected from deteriorationsand other influences resulting from the heating gases.

In the same manner electrically heated crucibles may be alsomanufactured.

The highly sintered, highly refractory metallic oxides have, moreover,the property of being also sufiiciently electrically insulating at hightemperatures. They are highly resistant to changes in temperature andhighly heat conductive so that a rapid heating is effected as well as auniform and good transfer of heat.

It will be understood from the above explanations that one of theimportant and essential. features of this invention is concerned with. aheating element and with a method for makingthe same, wherein anelectrical resistance consisting of a metallic member is embedded orenclosed gastight within a refractory casing of highly sintered metallicoxide.

The term fgas tight as herein used is understood to mean such acondition with reference to the effect of external gases upon theresistance element as can be objectively ascertained with known andapproved standard tests, for example, a test wherein the heating elementis immersed in a suitable solution under pressure for a predeterminedperiod of time. If the element has not absorbed any of the test solutionduring the test, it is then termed gas tight.

I claim as my invention:

1. A method of making electric heating ele ments consisting in providinga resistor with a temporary volatilizable coating, surrounding saidresistor with a sheath of highly refractory metallic oxide, heating saidsheath with the resistor and the coating in a furnace to a hightemperature to volatilize said coating, sintering the sheath to acompact and gas-tight body in tight engagement with said resistor, thethickness of said volatilizable coating being so chosen that the spaceoccupied thereby corresponds to the shrinkage of the sheath of highlyrefractory metallic oxide upon the sintering thereof so that after thesintering the sheath is in a completely gas-tight engagement with theresistor.

2. Ina method of making electric heating elements, the steps comprising,taking a metallic resistor, freely suspending said resistor in anabsorbing mold, pouring into said mold a plastic slip of highlyrefractory metallic oxide activated with diluted acid to form a sheathon said resistor, removing said resistor and said sheath from said moldafter removal of moisture therefrom and after solidifying said sheath,and heating said resistor and said sheath thereon in a furnace at atemperature exceeding 1600 0. whereby said sheath is sintered urito saidresistor to form therewith a permanent gas-tight bond.

REINHOLD REICHMANN.

